These fossils were referred to a new genus and species "Iemish listai" by naturalist Santiago Roth, who thought they might be the bones of the mythological iemisch of Tehuelche folklore.

Cave paintings made by indigenous peoples possibly depicting the animal have been found in the El Ceibo, which features a red coat with stripes and spots.

P. onca mesembrina was carnivorous and hunted a variety of large mammals, including the ground sloth Mylodon, horse Hippidion, and camelid Lama.

Near his farm in mountains in Última Esperanza Province, Chile, German explorer Hermann Eberhard came upon a cave bearing fossils of the ground sloth Mylodon, leading it to be dubbed "Cueva del Milodón".

[8][9] The next year, English paleontologist Arthur Smith Woodward referred the material to Felis, noting the bones' similarities to those of extant jaguars.

[10] Later in 1900, Hauthal collected additional felid elements from the cave including a partial skull, two mandibles, other postcranial remains, and isolated portions of skin.

[11] Later analysis by Argentine paleontologist Angel Cabrera in 1934 led him to conclude that some of the bones were from Neomylodon and that the name "Iemish listai" was invalid, as it was used to refer to these erroneously assigned fossils and iemisch.

In his 2020 thesis, researcher Nicholas Freymueller argued that this could be a case of Bergmann's rule, as traits correlated with larger body size were used to justify the synonymy in Chimento et al's study.

[13] In 2024, it was suggested that the validity of subspecific assignments on both P. o. augusta and P. o. mesembrina remains unresolved, since both fossil and living jaguars show a considerable variation in morphometry.

A similar trend of gigantism is observed in the North American subspecies Panthera onca augusta, which was around 15-20% larger than modern jaguars at around 190 kilograms (420 lb).

Roth described these portions in detail, noting that the skin from the face was reddish with hints of "shiny chestnut" while the limbs are dark overall with yellowish striping.

[11] Notably, a cave painting from El Ceibo archaeological locality in Santa Cruz Province bears an illustration matching this description.

The holotype of Panthera onca mesembrina preserved a shallow irregular pit on the lateral surface on the maxilla; this represents a puncture made by the canine of another jaguar, which healed during life.

[12][20]At Cueva del Milodon, fossils of the ground sloth Mylodon, horse Hippidion, and camelids have been found with tooth and scratch marks that match the teeth of P. onca mesembrina.

[12][23] The majority of sites where P. onca mesembrina has been found in Argentina and Chile are very open and arid, contrasting to the lush rainforest habitats of modern jaguars.

[26][27] This conclusion is backed by their diets, with P. onca mesembrina hunting grazing animals[21] whereas living jaguars prefer browsers like tapirs, anteaters, and peccaries.